Typically, optical imaging systems admit light from an object to create an image of the object. Often, it becomes necessary to rotate the image to facilitate viewing or analysis. Image rotation is used in aerial reconnaissance in which the imaging system is mounted in a moving aircraft for viewing ground objects. As the aircraft moves by the object the imaging system rotates to maintain the object in the field of view. If the object is not directly below the aircraft, the rotation of the imaging system causes the image to rotate with respect to the detection device receiving the image. In many cases, this image rotation is undesirable and must be corrected.
One possible means of correcting the image rotation is to rotate the detector. However, this is seldom feasible. Rotation of the object is also generally not feasible. Therefore, devices have been developed which are integrated into the imaging system for rotating the image in coordination with the rotation of the imaging system. The devices are located along the optical axis of the imaging system between the entrance aperture of the imaging system and the detector.
One such device is a K-mirror system. The K-mirror system consists of three mirrors oriented to reflect incoming light along a desired path. Light from the object enters the K-mirror system and impinges on each mirror in succession. Thus, for each point on the object, a path of light successively strikes a single point on each of the three mirrors and ends at a point in the image. A characteristic of the K-mirror system is that all five of these points, namely, the point on the object, the point on the image, and the points on each of the three mirrors, lie in a single plane.
In a stationary position, the output image of the K-mirror system is in an inverted orientation with respect to the object. As the K-mirror system is rotated, the output image rotates at twice the rotation rate of the K-mirror system. That is, for 90 degrees of K-mirror system rotation, the image rotates 180 degrees.
The K-mirror system has some fundamental limitations to its usefulness. The mirrors in the K-mirror system are oriented such that the light is reflected at obtuse angles. As a result, the output light from the K-mirror system possesses a high degree of polarization. In many applications, this polarization is undesirable. Also, as the K-mirror system rotates, it sweeps out a relatively large spatial volume, therefore consuming a large amount of space.